Effects of commercial whitening toothpaste containing hydrogen peroxide and citric acid on dentin abrasion and erosion

doi:10.21203/rs.3.rs-2779164/v1

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Research Article

Effects of commercial whitening toothpaste containing hydrogen peroxide and citric acid on dentin abrasion and erosion

https://doi.org/10.21203/rs.3.rs-2779164/v1

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published 01 Sep, 2023

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Background: Hydrogen peroxide (HP) and citric acid (CA), key contributors to toothpaste acidity, can lead to dental loss. This study is to compare the amount and pattern of abrasion or loss of dentin based on pH, buffering, and concentration of HP and CA in various commercial and experimental toothpastes after toothbrushing or immersion.

Methods: Bovine dentin specimens were randomly assigned to nine solutions. The prepared solutions included two commercial toothpastes (whitening toothpaste [WT] with HP and CA; conventional toothpaste [CT] without HP and CA), reference slurry (RS), two CA solutions (0.1 M, CAS1; 0.00005 M, CAS2), basic solution (0.2 M sodium phosphate dibasic [SPDS]), CA phosphate buffer solution (0.1 M SPDS and 0.05 M CA [CAPB]), HP solution (1.33 M; HPS), and distilled water (DW). After 10,000 brushings and immersion for 1 h, the amount of dentin loss and surface pattern were measured and observed using noncontact profilometry. Data were analyzed using an analysis of variance and the Tukey test as a post hoc analysis (p<0.05).

Results: WT with pH 5.0 had lower dentin abrasion than CT and RS after brushing but had higher dentin loss than both after immersion. While WT, CAS1, and CAPB surfaces exhibited comparable U-shaped patterns after brushing or immersion, a wedge shape was observed on CT and RS surfaces after brushing, with their surface patterns remaining intact after immersion. In addition, dentin abrasion, loss, and surface patterns of CAS2 and HPS, which had a pH of 5.0 like WT, did not significantly differ from those of DW after brushing and immersion.

Conclusions: After brushing or immersion, the CA concentration may affect the erosive pattern on the dentin surface more than the HP concentration included in WT. The amount of abrasion or loss of dentin after brushing or soaking can vary based on the composition, concentration, and buffer in the solution, even if the pH levels are similar. With or without pH buffering, prolonged use of WT containing HP and high CA concentrations may cause further dentin damage beyond simple brushing.

Whitening toothpaste

Hydrogen peroxide

Citric acid

Dentin abrasion

Dentin erosion

Consumers can now simply whiten their teeth at home using whitening toothpaste (WT), a popular product, to improve their smiles. WTs typically contain higher abrasive content and various types of abrasives and chemicals, such as hydrogen peroxide (HP) and optical brightening agents (e.g., blue covarine) [1]. WTs can be applied directly to the tooth surface using a single or combined strategy. It can remove plaque and stains on the tooth exterior or improve discoloration on the tooth interior when one brushes. Consequently, WTs can help improve aesthetics by making teeth cleaner and brighter than before [2].

HP is a representative chemical component included as an active ingredient in WTs among other additives. Although WTs have various HP concentrations depending on brands and products, the regulations regarding the maximum allowable HP concentration in WT may vary from country to country [3]. Due to its low molecular weight, HP can act directly on porous enamel and dentin and can also decompose into water and oxygen via an oxidation–reduction reaction. The free radicals produced during HP decomposition can break the double bonds of coloring molecules. In other words, they reduce the number of coloring molecules that absorb light, thereby increasing tooth-light reflectance. Therefore, teeth can appear brighter with improved whitening [1, 3].

The pH of WTs containing HP is generally below 5, which is significantly lower than that of conventional toothpaste and WTs without HP [4, 5]. Low pH can adversely affect the surface of dental hard tissues such as enamel and dentin by potentially causing erosion. Tooth erosion is the chemical dissolution of dental hard tissues by nonbacterial acids [6]. It may cause irreversible damage to these tissues, increased sensitivity, a greater risk of tooth fracture, and esthetic deterioration [6, 7].

WTs containing HP may have different pH levels depending on HP concentration. Notably, additives other than HP may also affect pH by adjusting it. WTs containing HP often include citric acid (CA) as a stabilizer. CA, which can adjust the pH of various products, is a commonly included ingredient in beverages, foods, pharmaceuticals, and dental products [8]. However, interactions between CA and dental hard tissues can lead to surface demineralization and probably increased susceptibility to dental abrasion [9, 10]. Dentin has a low modulus of elasticity and may be more susceptible to dental abrasion and erosion than enamel due to compositional and structural differences [11].

The commercial WTs incorporating HP have become prevalent more and more to satisfy consumer demand for at-home tooth whitening. They can be used regularly to achieve whiter teeth. In a previous study, WTs containing HP demonstrated a lower relative dentin abrasion–profilometry equivalent (RDA–PE) than conventional toothpaste and WTs containing sodium bicarbonate [4]. However, it is notable that WTs containing HP usually demonstrate U-shaped abrasion patterns compared to other toothpastes that commonly have multiple wedge or V-shaped abrasion patterns [4]. These U-shaped abrasion patterns resembled erosion patterns, as previously reported [12–14], and these results might be attributable to low pH levels.

HP and CA are key factors that contribute to toothpaste acidity and can lead to eroded patterns [9, 15]. Although numerous factors can affect the amount and feature of tooth surface abrasion or loss after brushing or immersion with WTs, studies regarding the effects of pH and acidic constituents of WTs, as well as their interaction, remain scarce. Therefore, this study assesses and compares the amount and pattern of abrasion or the loss of dentin surfaces resulting from the use of various commercial and experimental toothpastes, considering key factors such as pH, buffering, and HP and CA concentrations, after toothbrushing or immersion.

2.1. Bovine dentin specimens

Incisors were extracted from the mandibles of cows without defects such as caries or fractures. The central part of the crown was drilled using a perforator (punched diameter, 8 mm; YDM-13 mm, Yongsoo Precision, Daegu, Korea) while spraying water (Fig. 1a). Extracted teeth with a dentin thickness of at least 2 mm were selected to prepare the dentin specimens (Fig. 1b). The teeth were placed in an acrylic ring with outer and inner diameters of 30 and 12 mm, respectively, and the teeth were fixed by pouring self-curing resin (Vertex Self-Curing, Vertex, Zeist, Netherlands). The resin hardened for one week. The specimen surfaces were polished flat using a polishing machine (KDPI-330, KD Precision, Bucheon, Korea) and sandpaper (#220, 600, 1200, and 2000; R&B, Daejeon, Korea). A load of 300 g was applied to the specimen surface using a Vickers hardness tester (HMV-2, Shimadzu, Tokyo, Japan). An average value was calculated by measuring five points per specimen. Specimens corresponding to the values of 30–70, which is the Vickers hardness range for dentin, were obtained (Fig. 1c). Finally, specimens with concave surfaces were excluded before the experiment using noncontact surface profilometry (NV-1800, NanoSystem, Daejeon, Korea).

2.2. Preparation of experimental solutions

Two commercial toothpastes, which consists of one WT containing HP and CA and one conventional toothpaste (CT) without HP and CA, were prepared. Both commercial toothpastes were vigorously mixed in distilled water (DW) at a ratio of 1:1.6. To prepare a reference slurry (RS), a reference diluent solution containing 10% glycerin (99.5%, Shanghai Aladdin Biochemical Technology, Shanghai, China) and 0.5% carboxymethyl cellulose (CMC, Sigma-Aldrich, St., Louis, USA) was prepared. Calcium pyrophosphate (99.95%, Strem Chemicals, Newburyport, USA) and the diluent solution were mixed in a ratio of 1:5. CA solutions were prepared by dissolving or diluting CA (251275, Sigma-Aldrich, Steinheim, Germany) in DW to prepare 0.1 M (CAS1) and 0.00005 M (CAS2) CA solutions. A CA phosphate buffer (CAPB) was prepared by mixing 0.2 M sodium phosphate dibasic solution (SPDS; Junsei Chemical, Tokyo, Japan) and 0.1 M CA solution to a pH level of 5.0 [16]. The HP solution (HPS) was prepared by diluting 35% HP (18304, Sigma-Aldrich, St. Louis, USA) at a concentration of 1.33 M (4%), and the HPS concentration was checked using a digital refractometer (PR-50HO, ATAGO, Tokyo, Japan) with ± 0.5% accuracy. Lastly, DW was prepared.

The experimental solutions were measured using a pH meter (F-71, HORIBA, Kyoto, Japan) and pH electrode (9615S-10D, HORIBA) calibrated with three pH buffer kits (pH 4.0, 7.0, and 10.0, 502-S, HORIBA, Singapore). The temperature was 25℃ ± 0.5℃ when the pH was measured. Table 1 lists the ingredients, concentrations, and pH of the toothpastes and solutions used in the experiment.

Table 1

Detailed ingredients, concentrations, and pH of the toothpaste and solution used in the experiment.
Codes †	Productname /Manufacturer	Ingredients	pH	Fluoride (ppm)	Lot
WT	Vussen 28 ^a	Hydrogen peroxide 35% (containing 2.80%), Colloidal silicon dioxide, Glycerin, Sodium lauryl sulfate, Sodium metaphosphate, Sodium saccharin, Citric acid, L-Menthol, Purified water, Poloxamer 407, Polyethylene glycol 1500, Flavor, Hydroxyethylcellulose	5.011 ± 0.038	━	22H007
CT	Perioe new fresh alpha ^b	Sodium monofluorophosphate, Calcium carbonate, Glycerin, Sodium lauryl sulfate, Polyoxyethylene sorbitan monooleate, Amorphous sorbitol solution 70%, Saccharin sodium hydrate, Disodium dihydrogen pyrophosphate, Zinc acetate, Xylitol, Purified water, Carboxymethylcellulose sodium salt, Silica (TIXOSIL 43K), Flavor	7.774 ± 0.020	1000	FB23C
RS	Reference slurry	Calcium pyrophosphate, 10% glycerin, 0.5% carboxymethyl cellulose	6.850 ± 0.019	━	━
CAS1	Citric acid solution 1	0.1 M citric acid (1.92%)	2.026 ± 0.009	━	━
CAS2	Citric acid solution 2	0.00005 M citric acid (0.001%)	5.001 ± 0.030	━	━
CAPB	Citric acid phospate buffer	0.1 M sodium phosphate dibasic (3.58%) and 0.05 M citric acid (0.96%)	5.006 ± 0.003	━	━
SPDS	Sodium phosphate dibasic solution	0.2M sodium phosphate dibasic (7.16%)	9.077 ± 0.008	━	━
HPS	Hydrogen peroxide solution	1.33 M hydrogen peroxide (4%)	5.022 ± 0.018	━	━
DW	Distilled water	━	6.704 ± 0.054	━	━

^† WT = Whitening toothpaste, CT = Conventional toothpaste, RS = Reference slurry, CAS = Citric acid solution, CAPB = Citric acid phospate buffer, SPDS = Sodium phosphate dibasic solution, HPS = Hydrogen peroxide solution DW = Distilled water.

^a Osstem pharma, Ansan, Korea, ^b LG Household & Health Care, Cheongju, Korea.

2.3. Dentin abrasion and immersion process

For the abrasion test of the dentin surface, a window with 4 × 20 mm (width × height) was created using a 25 µm polyester tape (162.H421.25B, HaeSung Tape, Daejeon, Korea) to produce a reference surface that was not brushed on the specimen surface before testing. Each solution was poured after fixing eight specimens in the bath (n = 8). Toothbrushing was performed 10,000 times at a speed of 170 strokes per minute using an automatic brushing machine (RB118, R&B, Daejeon, Korea) and a three-row flat-bristle toothbrush (Name Brush T21, Guardian Angel, Suwon-si, Korea) with a bristle’s diameter of 178 µm (Fig. 1c). The load applied to the toothbrushes was 150 g.

In the immersion test, a window of the same size as the brushing process was created, and eight specimens were assigned to each solution (n = 8). Specimens were immersed for 1 h, equivalent to 10,000 brushing times with stirring.

After the abrasion and immersion tests, the tape was removed from each specimen and washed thoroughly with tap water.

2.4. Dentin surface measurement by noncontact profilometry

After brushing and immersion, the specimens were measured in continuous mode with a size of 2.304 × 1.728 mm (width × height) from the left reference surface to the right reference surface at 5x magnification using a noncontact surface profilometry. The analysis size was 1 mm on each reference surface and 4 mm exposed to brushing or immersion. The total width was 6 mm, and the height was 1.5 mm. Based on both reference planes, the average depth of the lost dentin by brushing or immersion was calculated using analysis software (NanoMap Ver. 3.5.17.7).

The RDA–PE value, which is the relative abrasion value of dentin, was determined using the following formula: the average depth of each solution divided by the average depth of RS multiplied by 100. In this study, 10,000 strokes of RS corresponded to RDA–PE 100 [4].

2.5. Observation, size, and content of abrasive

To observe the abrasives of CT, RS, and WT, which contained abrasives among the nine solutions, 1 g of commercial toothpastes (CT and WT) was added to 50 ml of DW and vigorously stirred to dissolve. The mixed solutions were centrifuged at 10,000 RPM for 15 min using a centrifuge (Avanti J-E, Beckman Coulter, Fullerton, USA) to separate the abrasive. The supernatant in the tube was discarded, and this process was repeated five times. Finally, the abrasives were rewashed with ethanol. The washed abrasives were completely dried at 37°C for three days using a drying oven (DO-49, Daeheung, Incheon, Korea). RS did not undergo this procedure. The abrasives were fixed to carbon tape on stubs and coated with platinum. They were observed at 3,000x magnification using a field emission scanning electron microscope (FE-SEM; Apreo S LoVac, Thermo fisher scientific, MA, USA). The abrasives were analyzed for elements in the visualized area using energy-dispersive X-ray spectroscopy (EDS; XFlash 6160, Bruker, Berlin, Germany) and analysis software (ESPRIT, ver. 2.1, Bruker).

An ultrasonic cleaner was used to disperse 30 mg of dried abrasives from CT, WT, and RS into DW for a size study of abrasives. The average particle size of each abrasive was analyzed in triplicate using a particle size analyzer (LA-950V2, HORIBA, Kyoto, Japan) (n = 3).

To determine the abrasive content of the CT, WT, and RS, 1 g of each solution was deposited in an alumina crucible. The crucible was covered with a lid and weighed using an electronic balance (HS220S, Hansung, Hwaseong, Korea). It was placed in an electric furnace (NEY 6-1350A, NEY, CA, USA) and heated at 2°C per minute to 600°C to incinerate the residue. The incinerator temperature was maintained at 600°C for 2 h and cooled slowly. After incineration, the changed weight of the crucible was measured, and the included abrasives were calculated as a weight percentage (wt%). The wt% of the abrasive was measured in triplicate each (n = 3).

2.6. Statistics

Each loss data on the dentin surface due to brushing or immersion was analyzed using statistics software (SPSS statistics v26.0, IBM, NY, USA). Before analysis, the normality test was checked using the Kolmogorov–Smirnov test. Data were analyzed using a one-way analysis of variance, followed by a Tukey test as a post-hoc test. The level of significance was 5%.

3.1. Dentin abrasion by toothbrushing

Table 2 lists the mean and standard deviation of the RDA–PE values and the average depth of the abrasion on the dentin surface. A significant difference was observed between groups based on the RDA–PE value and the average depth of abrasion (p < 0.001), with CAS1 presenting the highest values, while no significant difference was observed regarding CT. CAS1 and CT were significantly different from other groups (p < 0.05). RS, CAPB, and WT showed significant differences from other groups (p < 0.05), respectively. DW had the lowest RDA–PE value and average depth of abrasion; WT, CAPB, RS, CT, and CAS1 differed significantly (p < 0.05), and SPDS, CAS2, and HPS did not.

Table 2

Means and standard deviation of the RDA–PE and average depth of dentin by toothbrushing.
Solution	RDA-PE	Average depth of abrasion (㎛)
CAS1 (pH 2.0)	162.0 ± 30.0 ^a	55.61 ± 10.14 ^a
CT (pH 7.8)	155.0 ± 22.0 ^a	53.22 ± 7.45 ^a
RS (pH 6.9)	100.0 ± 29.0 ^b	34.29 ± 10.11 ^b
CAPB (pH 5.0)	62.0 ± 8.0 ^c	21.24 ± 2.66 ^c
WT (pH 5.0)	29.0 ± 5.0 ^d	10.10 ± 1.71 ^d
HPS (pH 5.0)	3.0 ± 0.4 ^e	1.10 ± 0.15 ^e
CAS2 (pH 5.0)	3.0 ± 0.7 ^e	0.95 ± 0.24 ^e
SPDS (pH 9.1)	1.0 ± 0.5 ^e	0.41 ± 0.19 ^e
DW (pH 6.7)	1.0 ± 0.3 ^e	0.35 ± 0.12 ^e

The RED-PE values and average depth of abrasion was described in descending order

Different letters imply significant differences between groups (p < 0.05; n = 8).

Figure 2 shows the two-dimensional (2D), three-dimensional (3D), and optical images of abrasion patterns on the dentin surface based on the solution type. A multitudinous wedge or V-shaped abrasion pattern was typically observed on the CT and RS surfaces. CAS1, CAPB, and WT exhibited U-shaped abrasion patterns in comparison to CT and RS. The abrasion patterns on the CAS2, SPDS, HPS, and DW surface showed minor abrasion compared to the reference surface.

Code: WT (Whitening toothpaste; Vussen 28); CT (Conventional toothpaste; Perioe new fresh alpha); RS (Reference slurry); CAS1 (0.1 M, 1.92% citric acid solution); CAS2 (0.00005 M, 0.001% citric acid solution); CAPB (0.1 M, 3.58% sodium phosphate dibasic and 0.05 M, 0.96% citric acid; Citric acid phospate buffer); SPDS (0.2M, 7.16% Sodium phosphate dibasic solution); HPS (1.33 M, 4% hydrogen peroxide solution); DW (Distilled water)

3.2. Dentin loss by immersion

Table 3 lists the mean and standard deviation of the average depth of the dentin surface after immersion for 1 h in each solution. The mean depth values significantly differed between groups (p < 0.001). CAS1 had the highest depth value, and the other groups significantly differed (p < 0.05). CAPB significantly differed from all groups, excluding WT. DW had the lowest depth value, CAS1, CAPB, and WT groups differed significantly (p < 0.05), and RS, SPDS, CT, CAS2, and HPS did not.

Table 3

Means and standard deviation of the average depth of dentin surface by immersion.
Solution	Average depth of dentin (㎛)
CAS1 (pH 2.0)	23.44 ± 3.29 ^a
CAPB (pH 5.0)	3.21 ± 0.91 ^b
WT (pH 5.0)	2.57 ± 0.65 ^b
HPS (pH 5.0)	0.38 ± 0.11 ^c
CAS2 (pH 5.0)	0.33 ± 0.13 ^c
CT (pH 7.8)	0.30 ± 0.05 ^c
SPDS (pH 9.1)	0.30 ± 0.07 ^c
RS (pH 6.9)	0.27 ± 0.07 ^c
DW (pH 6.7)	0.26 ± 0.06 ^c

The average depth of dentin was described in descending order.

Different letters imply significant differences between groups (p < 0.05; n = 8).

Figure 3 shows the 2D, 3D, and optical images on the dentin surface after immersion in the solutions. On the dentin surface in touch with CAS1, CAPB, and WT, a loss of U-shape and surface change were detected in comparison to the reference surface. The dentin surfaces of the remaining groups were typically flat, like the reference surface.

3.3. Observation, size, and content of abrasive

The abrasives of CT, RS, and WT were observed in various sizes and had polygonal or circular shapes, as shown in the FE–SEM images (Fig. 4a). Most of the abrasives were observed within 30 µm in all three groups. The abrasives in WT had relatively smaller abrasive than CT and RS. The elements of the abrasives were analyzed in the observed area by EDS, and the abrasives corresponding to each were matched (mapping) to FE–SEM images (Fig. 4a). Silica (blue color) in WT, silica and calcium (red color) in CT, and calcium and phosphorus (green color) in RS were mainly detected.

As determined by the analysis of particle size distribution, the average abrasive sizes of WT (5.94 ± 0.27 µm) were smaller and more homogeneous than those of CT (7.06 ± 0.61 µm) and RS (9.75 ± 0.18 µm) and were similar to those visualized by FE–SEM (Fig. 4b).

After incineration at 600°C in the furnace, the wt% of abrasive in WT, RS, and CT was 3.57 ± 0.23%, 16.60 ± 0.10%, and 18.27 ± 0.06%, respectively (Fig. 4c).

Code: WT (Whitening toothpaste; Vussen 28); CT (Conventional toothpaste; Perioe new fresh alpha); RS (Reference slurry)

Using noncontact surface profilometry, the amount of dentin abrasion or dentin loss was measured by RDE-PE after 10,000 brushings or immersion for 1 h and was compared among various solutions. Among them, CAS2, CAPB, and HPS with pH of 5.0 have the similar the level of pH to WT containing HP though they have different ingredients. In addition, CAS1 and SPDS were assessed because they are used to prepare CAPB, which has higher concentration of CA than CAS2, even if it has the same level of pH with CAS2. The pH of WT (5.011 ± 0.038), which was below 6.0, the critical pH of dentin [17, 18], was sub-acidic in contrast to CT, RS, SPDS, and DW in the present study (Table 1).

A solution with a relatively low pH may cause demineralization and loss of dentin by erosion [19], although it may be advantageous for preventing HP decomposition [20]. According to a previous study, HP oxidation was mitigated when CA was added to a 0.3% HP solution adjusted to a pH range of 2–6 rather than an alkali solution of pH 8.0 or higher under harsh conditions of 40℃ for six weeks [21]. CA can lower the pH of HP solutions and may be used as a stabilizer due to the slow decomposition of HP. Low pH can act as a preservative by preventing bacterial growth [22]. In other words, adding CA to WTs containing HP can be beneficial for maintaining HP concentration and long-term storage of dentifrice. CA can directly dissolve tartar, which can help HP to immediately act on teeth, functioning as an accelerator for tooth whitening [23]. Compared to other acids, CA is readily available, has a low pKa, and is inexpensive [21].

Solutions with a low pH can be pH modified or buffered with various additives to reduce tooth loss [24, 25]. In the current study, CAPB was mixed with CAS1 (pH 2.026 ± 0.009) and SPDS (pH 9.077 ± 0.008) to achieve a pH of 5.0 similar to WT; as a result, the pH of CAPB was corrected and buffered (Table 1). For all solutions, the amount of abrasion on the dentin after brushing was determined by the RDA–PE value.

The RDA–PE value, which is the relative abrasion value of dentin by the profilometry method, can quantify the degree of dentin abrasion compared with the RS based on ISO 11609. Profilometry is a well-established method for evaluating dentin abrasion or erosion, and the shape of the exposed surface can be visualized [25]. ISO 11609 considers the RDA–PE value of RS to be 100 and limits the maximum RDA–PE value to 250 [4].

A low pH can soften the dentin surface, causing more abrasion [26]. Among all groups, CAS1 had the lowest pH (2.026 ± 0.009), and the RDA–PE value (162.0 ± 30.0) was the highest after toothbrushing, although an abrasive was not included in the solution (Tables 1 and 2). The overall U-shaped abrasion pattern of CAS1 may be due to the large amount of dentin being easily removed by brushing (Fig. 2). As the pH of CAPB was buffered and adjusted to the 5.0 level, the RDA–PE value of CAPB (62.0 ± 8.0) was reduced by more than half (about 62%) compared to that of CAS1, and the size of the abrasion also decreased. Interestingly, despite the lack of abrasives, CAPB had a higher RDA–PE value than WT containing abrasives (29.0 ± 5.0); its RDA–PE value significantly exceeded those of CAS2 (3.0 ± 0.7) and HPS (3.0 ± 0.4) that lacked abrasives and had a pH of 5.0. These findings suggest that, even at equal pH levels of 5.0, RDA–PE values can differ depending on the CA concentration or other ingredients present in the solution.

The amount and pattern of dentin abrasion may be closely related to factors such as the type, content, characteristics (e.g., size, shape, hardness, and homogeneity) of the abrasive, other additives, and pH in the solution during brushing [27–31]. Among the three solutions containing abrasives in CT, RS, and WT, U-shaped abrasion patterns were typically observed on the abrasion surface of WT similar to those of CAS1 and CAPB (Fig. 2), whereas the abrasion patterns of CT and RS often exhibited a multiple V or wedge shape. However, WT had a significantly lower RDA–PE value (29.0 ± 5.0) than RS (100.0 ± 29.0) and CT (155.0 ± 22.0). WTs may raise concerns regarding increased abrasion on the dentin surface because they are often conjectured to contain more abrasives than conventional toothpastes [32–34]. However, this study showed that WT containing HP (3.57 ± 0.23%) had significantly lower abrasive content than RS (16.60 ± 0.10%) and CT (18.27 ± 0.06%). Although the type, size, and content of the abrasive in this study may have varied (Fig. 4), a complex mechanism of factors including the properties of the abrasive, other ingredients containing CA, and low pH during toothbrushing may have induced differences in RDA–PE values and abrasion patterns among WT, CT, and RS.

After immersion in the solutions, CAS1 (23.44 ± 3.29 µm) with pH 2.0 caused 90 times more dentin loss than DW (0.26 ± 0.06 µm) without brushing, and the surface of dentin in contact with the solution presented U-shaped damage (Table 3 and Fig. 3). CAPB (3.21 ± 0.91 µm) buffered to pH 5.0 had its value reduced by approximately 86% compared to CAS1. WT (2.57 ± 0.65 µm) containing HP and CA caused approximately 10 times more dentin loss than DW, and the dentin surface exposed to the solution was similar to CAS1 and CAPB (Fig. 3). Here, note that HPS (0.38 ± 0.11 µm), which contained a 4% concentration of HP higher than the 2.8% concentration of HP included in the WT, CAS2 (0.33 ± 0.13 µm), which contained a relatively low CA concentration, and the solutions had a pH of 5.0. However, neither solution induced significant dentin loss and surface changes compared to DW, indicating that factors such as CA concentration may have contributed more to the damage on the dentin surface than the low pH itself that the HP concentration causes in this case.

pH is usually a measure of the relative amounts of free hydrogen and hydroxyl ions and can be perceived as relative acidity in a solution [35]. However, pH trends may not be similar to acidity trends and may not be synonymous [36, 37]. Loss of dentin surface may vary depending on factors such as acid properties (e.g., type, concentration, and chelating effect), pH, buffering capacity, or amount of titrate acid [24, 25, 38]. Previous studies have demonstrated that a 6% CA solution was more effective than a 3% HP solution in removing the smear layer of dentin [39]. The solutions of three CA concentrations (0.07%, 0.25%, and 1.00%) with a similar pH range of 3.60–3.77 buffered with sodium citrate caused more dentin loss with increasing CA concentration [40]. In addition, the effect of removing dentin smears for 1 min was not significantly different between solutions with a pH of 2.0 or less plus a high concentration of 25% or 50% CA and the pH 6.0 level group buffered with sodium hydroxide. In other words, a high CA concentration in solutions could cause significant dentin surface loss even if the pH was buffered [41]. These results corroborate our findings [39–41].

Because commercial WTs have lower HP concentrations than professional whitening treatments, teeth may require long-term brushing in order to achieve the desired whitening effect [42]. In the present study, 10,000 brushings could clinically be equivalent to approximately one year of cumulative brushing per tooth [43]. Even though the remineralization process and intermittent brushing in real life are not taken into consideration in the present study, long-term use of WT containing HP and high CA concentrations may cause additional dentin damage other than brushing, with or without pH buffering. Information about the acid used in commercial WT may not be provided due to trade secrets in the packaging if not compulsory [44]. It may contain various acids in different concentrations, and the pH of WTs may be buffered with other additives, necessitating further study.

In this study, commercial WT containing HP and CA with weak acidity caused significantly less dentin abrasion than CT and RS. However, it can cause more dentin loss after immersion without toothbrushing. CA concentration may affect the erosive pattern on the dentin surface more than HP concentration included in WT after toothbrushing or immersion. The amount of abrasion or loss of dentin after brushing or soaking can vary by several orders of magnitude based on the composition, concentration, and buffer in the solution, even if the pH levels are similar, and the effect on the dentin surface may be deceptive.

Whitening toothpaste

Hydrogen peroxide

Citric acid

RDA-PE

Relative dentin abrasion–profilometry equivalent

Conventional toothpaste

Distilled water

Reference slurry

CAS1

Citric acid solution1

CAS2

Citric acid solution2

CAPB

Citric acid phosphate buffer

SPDS

Sodium phosphate dibasic solution

HPS

Hydrogen peroxide solution

FE-SEM

Field emission scanning electron microscope

EDS

Energy-dispersive X-ray spectroscopy

wt%

Weight percentage

Two-dimensional

Three-dimensional

Ethics approval and consent to participate

We confirm that all methods were carried out in accordance with relevant animal guidelines and regulations. According to the Seoul National University Institutional Animal Care and Use Committee (SNUIACUC), which is our animal ethics approval institution, it stated that experiments with tissues of non-living animals by slaughter are legally possible without going through committee deliberation and approval procedures. Also, we legitimately purchased bovine teeth from the slaughterhouse and obtained permission for use from the owner.

Consent for publication

“Not applicable”

Availability of data and materials

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.

Competing interests

The authors declare that they have no competing interests

Funding

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2021R1l1A204851611).

Authors' contributions

JH contributed to the conceptualization of the experiments and writing the draft. SY performed data research, paper review, and editing. YS contributed to the supervision and acquisition of funds. All authors read and approved the final manuscript

Acknowledgements

“Not applicable”

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No competing interests reported.

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published 01 Sep, 2023

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Effects of commercial whitening toothpaste containing hydrogen peroxide and citric acid on dentin abrasion and erosion

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Journal Publication

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Abstract

Figures

1. Background

2. Materials And Methods

2.1. Bovine dentin specimens

2.2. Preparation of experimental solutions

2.3. Dentin abrasion and immersion process

2.4. Dentin surface measurement by noncontact profilometry

2.5. Observation, size, and content of abrasive

2.6. Statistics

3. Results

3.1. Dentin abrasion by toothbrushing

3.2. Dentin loss by immersion

3.3. Observation, size, and content of abrasive

4. Discussion

5. Conclusion

Abbreviations

Declarations

References

Additional Declarations

Status:

Journal Publication

Version 1